The historical trend in the design, development and commercialization of the vapor refrigeration cycle system has involved the use of high vapor pressure (therefore, high density) refrigerants in order to permit the use of smaller size conventional compressors.
For small buildings, mobile vehicle, aircraft and other medium capacity heating and/or cooling applications, high vapor pressure refrigerants have been used because the limited displacement of conventional compressors which satisfy practical size limitations requires high density gases to provide adequate cooling capacity for such applications.
Leakage of refrigerant gas from compressors in systems using high vapor pressure refrigerants, however, raises very serious problems. Home air conditioning systems, for example, are expected to operate for many years without loss of refrigerant. However, exposed rotary seals of the compressors are unable to contain high pressure refrigerants for the length of time required of modern home air conditioning systems.
This has led, in the case of home air conditioning systems, for example, to the requirement for totally hermetic systems wherein the electric motors are hermetically sealed within a tank along with the compressor.
Ceramic or glass electrical feed-throughs are required to pass electrical energy to the electric motor inside. Further, the refrigerant gas is used to cool the motor. Such hermetic designs have several major disadvantages:
(a) in the event of a compressor failure, the entire hermetic system must be replaced even though the motor may be fully functional;
(b) if the electric motor fails, the entire hermetic system must be replaced and, due to the electrical failure, the entire air conditioning system will be contaminated and have to be flushed and cleaned, thereby using a considerable amount of fluorocarbons and other cleaning and purging materials to effect the cleaning process; in some cases, depending upon the mode of failure, entire subcomponents, such as heat exchangers, receiver dryers and expansion valves, must be replaced;
(c) due to the fact that the electric motor is cooled by the refrigerant gas, the condenser heat exchanger must be up-sized to withstand the additional thermal load imposed by the inefficiencies of the electric motor; this adds further to the cost of the hermetic system; and
(d) the hermetic tank adds further costs to hermetic systems.
When leakage of refrigerant can be tolerated from the viewpoint of service, air conditioning and refrigeration systems employ the so-called "open system". In this configuration, a compressor with exposed rotary seals is used without a hermetically sealed electric motor. In this case, the components are discrete and separate and can, therefore, be serviced separately in the event of failure or the requirement for recharge of refrigerant. However, because the refrigerants are of high pressure, the inevitable leakage of the rotary compressor shaft seals necessitates relatively frequent recharging of the system.
In the case of automotive air conditioning systems, it has been found that the use of hermetic systems is not practical because of the inefficiencies and expense involved in having a separate high capacity alternator operating a hermetically sealed electric motor driven compressor system. Weight and complexity are particular disadvantages of hermetic systems in automotive air conditioning applications. Therefore, commercially available automotive systems are of the "open system" type and the problem of relatively frequent re-evacuation and recharging are tolerated.
Also refrigeration and air conditioning system efficiencies are limited by compressor efficiencies. Significant improvements in system efficiencies, therefore, must come from raising compressor efficiencies. However, conventional compressors requiring rotary seals to prevent leakage of high vapor pressure refrigerants have attained nearly maximum achievable efficiencies with known technology. Specifically, the typical home air conditioning compressor has an efficiency of approximately fifty percent.
The problem of refrigerant gas leakage in open systems can be substantially alleviated by using low vapor pressure refrigerant. Heretofore, low vapor pressure refrigerant has been used in refrigeration systems operating in vapor refrigeration cycles for large capacity cooling or heating applications, such as for large water chillers in building cooling systems or for shipboard environmental control systems. Such large water chillers or environmental control systems have typically employed large conventional centrifugal compressors to provide adequate cooling capacity for the application involved. It has not been found feasible in the past to use low vapor pressure refrigerant in refrigeration systems for medium capacity refrigeration or heating applications, such as for residential, automotive or aircraft heating and/or cooling, because conventional centrifugal compressors or piston compressors are generally too large to be practical in the range of displacements required for such applications. Large machines are relatively inefficient when using low density refrigerants compared with the mass flow produced and are relatively expensive to manufacture. In addition, their large size precludes the use of such machines as a practical matter in most such medium capacity applications because of space limitations.
Recently, a rotary vane compressor has been developed which is more efficient compared with conventional compressors due, primarily, to physical and dynamic constraints imposed on the vanes to eliminate sliding friction between the vane tips and stator wall by maintaining clearance at the vane tips. The term "constrained rotary vane compressor" is intended to mean rotary vane compressors having means for constraining the vanes to minimize friction between the vane tips and stator wall, compressors of this type being disclosed for example, in commonly assigned U.S. Pat. No. 4,410,305 and U.S. Pat. No. 4,299,097.
It has been found also that constrained rotary vane compressors may be constructed with a low "space" factor, i.e., ratio of external size to displacement. Another feature of constrained rotary vane compressors is that operational efficiency may be enhanced by providing large radial flow suction and discharge port areas resulting in improved port flow characteristics. Another advantage of constrained rotary vane compressors includes the potential of relatively low manufacturing cost per unit of displacement compared with conventional compressors because of mechanical clearances among the moving parts due to the large swept volume and low pressure differences between adjoining chambers within the machine.